When Tamerlan Tsarnaev came to renew his passport and visit his father last year, he spent time helping remodel a room into a prospective perfume shop and apparently slept a lot, says a family friend. But the surroundings were anything but quiet.

Violence that has racked the North Caucasus region for years has spread to the republic of Dagestan and reached its capital, Makhachkala, which can often seem a city under siege. Checkpoints are a common sight, as are patrolling police with body armour and assault rifles.

The battle between authorities and militants espousing separatist or radical Islamist ideals has taken on characteristics of an exchange of vendettas. One day, a body is found in a ditch with signs of torture and gunshot wounds to the head. The next, a police checkpoint is attacked.

Tsarnaev, 26, a suspect in the Boston Marathon bombings, was killed early on Friday in a confrontation with police. His 19-year-old brother, Dzhokhar, who is also a suspect, was captured hours later.

The brothers had lived in Dagestan before moving to the United States about a decade ago. Their mother acknowledged in an interview with a Russian television station that Tamerlan had become more religious and had been interviewed by the FBI about his views the year before he returned to the region for six months last year. His visit has led to speculation linking them with the Caucasus Emirate movement led by feared warlord Doku Umarov. However, a website used by Russia's North Caucasus rebels yesterday denied any link to the attacks in Boston.

"The command of the Vilayat Dagestan mujahedeen … declares that the Caucasus fighters are not waging any military activities against the United States of America," the Kavkazcenter.com website said. "We are only fighting Russia, which is not only responsible for the occupation of the Caucasus, but also for monstrous crimes against Muslims."

Tamerlan Tsarnaev's family also continued to reject the possibility that he carried out the attack. And it was unclear what effect, if any, his visit to the North Caucasus had.

He arrived in Makhachkala in January last year to renew his Russian passport, according to his father - something that can be done without leaving the US by applying at a Russian consulate. He was not registered with any of the consulates in the US, Russian officials said.

"The boy wasn't gloomy, but he kept quiet most of the time as he was helping his father," said Vyacheslav Kazakevich, an electrician who described himself as a long-time family friend. "[The father] was working hard to turn a room on the ground floor of a residential house into a perfume shop and open this new business this year."

Kazakevich said Tamerlan Tsarnaev had a beard and appeared to lead a quiet life while there. He was frequently absent from work during the mornings, the electrician said, with his father shrugging and saying he was still asleep. The would-be perfume shop was a ground-floor apartment in an old five-storey Soviet-style block of flats.